Question

我会知道是否有可能在带有空间的外键上创建一个空引用。

现在我的数据库结构是这样的：

@Entity(tableName = "A")
class A {
    @PrimaryKey(autoGenerate = true)
    public long id;
}

@Entity(tableName = "B")
class B{
    @PrimaryKey(autoGenerate = true)
    public long id;
}

@Entity(tableName = "C", foreignKeys = {@ForeignKey(entity = A.class, parentColumns = "id", childColumns = "foreign_id_a"), @ForeignKey(entity = B.class, parentColumns = "id", childColumns = "foreign_id_b")})
class C{
    public long id;
    public long foreign_id_a;
    public long foreign_id_b;   
}

我希望能够插入以下对象：

C(id=1, foreign_id_a=1, foreign_id_b=1)
C(id=1, foreign_id_a=null, foreign_id_b=1)
C(id=1, foreign_id_a=1, foreign_id_b=null)

但是前一个具有空值的插入会出现以下错误： FOREIGN KEY约束失败（SQLite代码787 SQLITE_CONSTRAINT_FOREIGNKEY）

有没有办法使之成为可能？

Answer 1

是的，只需将外键类型从long更改为Long。

在Java中，long类型不能为null，因此Room将此列生成为NOT NULL。另外，C类未指定@PrimaryKey。

@Entity(tableName = "C", foreignKeys = {@ForeignKey(entity = A.class, parentColumns = "id", childColumns = "foreign_id_a"), @ForeignKey(entity = B.class, parentColumns = "id", childColumns = "foreign_id_b")})
class C{
    @PrimaryKey
    public long id;
    public Long foreign_id_a;
    public Long foreign_id_b;   
}

房间内可空外键

1 个答案: